Developing unit and developing method

ABSTRACT

A developing unit includes a housing enclosing a mesh belt stretched across first and second rollers. The belt passes between a pair of friction plates that can be used to tribocharge toner particles carries on the mesh belt. Charging occurs over the large width of the entire mesh belt and the friction plates. Toner is transported on the surface of the mesh belt to a developing roller which, in turn, transports the toner to the surface of a photoconductor. The configuration of this developing unit reduces the loading associated with the tribocharging operation and the belt transport system reduces the volume of the developing unit.

This application claims priority from U.S. provisional patentapplication Ser. No. 60/21,393, filed Jul. 9, 1996 and from U.S.provisional patent application Ser. No. 60/21,377, filed Jul. 9, 1996.This application is related to U.S. patent application Ser. No.08/888,221, filed on Jul. 3, 1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to electrophotographic processes and, moreparticularly, to an apparatus and a method for developing images in anelectrophotographic process.

2. Description of the Related Art

Most computer systems, and most particularly small or "personal"computer systems, have been making increasing use of colors in thedisplay of information. The increasing use of color in personalcomputers reflects both the more ready availability of low price colordisplays and the more powerful and faster microprocessors that are usedin personal computers. The technology for providing printed coloroutput, i.e., "hard copy" output such as color printouts on paper,plastic or other materials, has not kept pace with the technology forthe color display of information. Presently, the options available forcolor hard copy output either do not present sufficiently high qualityoutput or are undesirably expensive for home or small office use.Examples of this conventional color output technology include ink jetprinting, whether using liquid or solid inks, as well as a few differentimplementations of color electrophotographic printing. Ink jet printingis comparatively inexpensive, at least when using liquid inks, but tendsto be slow and it is difficult to obtain acceptably high quality outputusing liquid ink jet printing technology.

Color electrophotographic printing is commercially available, but tendsto be expensive and slow. For example, multipass colorelectrophotographic printing is a process by which multiplephotoconductor exposures and multiple developing processes are used tocreate a multicolor image on the surface of a paper sheet. In essence,conventional multipass color electrophotographic printing consists ofrepeated application of single color or monochrome electrophotographicprinting processes using different colors for each successiveapplication or pass through the multipass printer. Traditionalmonochrome (black and white) electrophotographic printing forms an imageon the optically active surface of a photoconductor by exposing thephotoconductor using a laser or an equivalent high intensity lightsource. Before exposure, a uniform charge distribution is provided overthe surface of the photoconductor and, after exposure, a charge patterncorresponding to the exposure image to be printed is present on thesurface of the photoconductor. The latent image corresponding to thecharge pattern on the surface of the photoconductor is converted to aphysical image by a developer which adheres charged toner particles tothe charge pattern on the photoconductor in a pattern corresponding tothe latent image. The toner image is transferred onto a paper sheetusing an electrostatic transfer process and then fusing is performed tofix the toner image on the paper sheet. In a multipass color laserprinter or other similar electrophotographic apparatus, this printingprocess would be repeated several times.

FIG. 1 illustrates a conventional, multipass color electrophotographicapparatus which is assumed to be a laser printer for the purposes ofthis discussion. The multipass laser printer of FIG. 1 includes aphotoconductive drum 10, a charger 14 for creating a uniform chargedistribution on the surface of the photoconductive drum 10, a laser beamscanning unit 16 for exposing the surface of the photoconductive drumwith an optical image, a developing device 20 including a plurality ofsingle color developing units 22, 24, 26, 28 for developing a latentimage, a transfer charger 30 for applying a transfer electrical field,and a fuser 40 for fixing an image onto a recording medium 32 such as apaper sheet. The photoconductive drum 10 is generally a metal cylindercovered by an optically active material 12 known as the photoconductor.The photoconductor generally is highly insulative in the dark whiledeveloping a substantial level of conductivity under illumination. Thus,the photoconductor 12 can hold a charge on its surface in the dark, butcharge on the surface of the photoconductor is discharged underillumination.

In operation, a uniform charge is applied to the surface of thephotoconductor 12 at the beginning of each pass of the multipass colorprinting process. Charging of the photoconductor surface is accomplishedwith charger 14, which typically uses corona charging or a similartechnique to provide charge to the surface of the photoconductor 12.After the charging operation, the photoconductor 12 has on its surface auniform charge distribution corresponding to a voltage of ±600˜±800 V.When the photoconductor 12 is exposed by the laser beam scanning unit16, a laser beam 18 directed by the scanning unit 16 illuminates aspecified area of the photoconductor 12 in accordance with an imagemodulation pattern generated by a controller (not shown). The voltage onthe portions of the photoconductor 12 illuminated by the laser beam 18is discharged to approximately 0˜±150 V.

Multipass color laser printing is accomplished by successively formingon the surface of the photoconductor 12 successive monochrome images sothat, when all of the monochrome images are combined together on thephotoconductor 12, the combined image provides an acceptable colorimage. Typical multipass color electrophotographic strategies use fourprinting passes, with each successive pass applying a different opticalimage to the photoconductor corresponding to a different monochromeimage component. Each successive image is developed after the exposureportion of the pass with a developer having the appropriate color oftoner corresponding to that monochrome portion of the image. To effectthis strategy, it is necessary to provide four different developingunits 22-28 as shown in FIG. 1 having four distinct colors of toner tobe applied in successive ones of four different passes. Thus, fourdeveloping units 22-28 corresponding to yellow (Y) toner, magenta (M)toner, cyan (C) toner and black (K) toner respectively are provided forthe FIG. 1 printers. The reproduced image is therefore made up of aplurality of colors applied in varying concentrations to achieve variousgray levels.

In a first pass of the multicolor printing process of FIG. 1, the laserbeam scanning unit 16 exposes the surface of the photoconductor withmodulated laser light 18 to create a first latent image componentcorresponding to the first monochrome component of the image to beprinted. After the photoconductor 12 is exposed with the first latentimage component, the first component of the image pattern is developedusing a first developer 22, described in greater detail below, toprovide a first color of toner to the surface of the photoconductor.After the first monochrome component of the color image to be printedhas been provided on the surface of the photoconductor, a second pass isperformed to provide a second monochrome component of the color image tobe printed. The photoconductor 12 on the drum is charged to provide anew uniform charge distribution on the photoconductor. The laser beamscanning unit 16 then scans the laser beam 18 over the surface of thephotoconductor to expose the photoconductor 12 with a second latentimage component. A second color of toner is applied by the seconddeveloping unit 24 so that it adheres to the photoconductor 12 in apattern corresponding to the second latent image component and overliesthe first toner image. This process is repeated for the third and fourthcomponents of the image, using the third and fourth developing units 26and 28, respectively, to provide four different overlaid monochrometoner images on the photoconductor. The four color toner image is thentransferred onto the surface of a recording medium 32 such as a papersheet at the transfer charger 30 and the toner image is fused to therecording medium 32 at fuser 40. To accomplish fusing, the recordingmedium 32 is passed between the heating roller 42 and the pressingroller 44 that make up the fuser 40. A heater, such as a halogen lamp,is provided in the heating roller 42 to heat a surface of the roller toa predetermined high temperature sufficient to at least soften thedeveloper, when combined with the pressure applied by the pressureroller 44. The high temperature and pressure between these two rollerscause the toner to melt and to be fixed onto the recording medium 32,thereby forming a color image on the recording medium.

For the four pass color laser printer illustrated in FIG. 1, fourdeveloping processes are needed to create an image. Because the FIG. 1printer essentially requires four complete and independent printingprocesses to create an image, the FIG. 1 printer is about four times asslow as a conventional monochrome laser printer. As such, the colorimage reproduction rate for the FIG. 1 printer is generally unacceptablyslow.

FIG. 2 is a schematic view showing a different conventionalimplementation of a color electrophotographic apparatus. The "tandem"color electrophotographic apparatus of FIG. 2 is similar to theapparatus of FIG. 1 in that the printed color image is built up over thecourse of repeated distinct monochrome printing processes. For the FIG.2 apparatus, however, the four distinct printing processes are performedin series on four distinct drums 10 having associated chargers 14, laserbeam scanning units 16, four distinct developers 22, 24, 26, 28 carryingfour different colors of toner, and four transfer chargers 30. Operationof the FIG. 2 apparatus is generally similar to that of the FIG. 1apparatus, with the primary exception that each component of the latentimage is formed on different photoconductive drums by dedicated laserbeam scanning units in the FIG. 2 apparatus. Like elements in FIG. 2 arerepresented by like reference numerals in FIG. 1. A first color image isformed and then transferred to a recording medium 32 from the firstphotoconductive drum 10, and then second, third and fourth color imagesare successively transferred from the second, third and fourthphotoconductive drums in sequence. As illustrated, the FIG. 2 structurecan accommodate a linear transport path for the recording medium 32 sothat the four color components of the image can be transferred to therecording medium in a single transport operation. Thereafter, a fuser 40fixes the resultant four component image on the recording medium 32. Theapparatus of FIG. 2 is advantageous in that the reproduction ratethereof is much higher than that of the apparatus shown in FIG. 1 sincethe four developing processes can proceed simultaneously. The tandemcolor electrophotographic apparatus of FIG. 2 has undesirablecharacteristics, however. Although the FIG. 2 apparatus is improved overthe FIG. 1 apparatus, there remains in the FIG. 2 apparatus a difficultyin obtaining a desirable level of registration between the successiveimages transferred onto the paper sheet or other recording medium due tothe need to align four different drums with the recording medium for thefour successive toner transfer operations. More importantly, the FIG. 2apparatus can be undesirably large and expensive due to the need toprovide multiple complete print stations.

A different implementation of a color electrophotographic apparatus isillustrated in FIG. 3. The illustrated color laser printer provides fourcolor components for a printed color image while using a single passprinting operation. The FIG. 3 laser printer includes a photoconductivedrum 10 with four printing stations, each including a charger 50-56, alaser beam scanning unit 58-64 and a developer 60-72, all arrangedaround the circumference of the drum. At the first printing station, thedrum 10 is charged to an initial uniform voltage by the charger 50, thedrum is exposed according to a first component of the image to provide afirst latent image component, and the first latent image component isdeveloped by the developer 66 to produce a first toner pattern on thesurface of the drum. The second printing station repeats this processusing a second charger 52 to reproduce a uniform charge distributionover the surface of the drum 10, including over the portions of the drumcovered by the first toner image. As second toner image is created onthe surface of the drum and is overlaid with the first toner image.Third and fourth color image components are created on the surface ofthe drum 10 as the drum rotates through the third and fourth developingstations to provide a four component, four color toner image on thesurface of the drum. The four color toner image is then transferred ontothe recording medium 32 by transfer charger 30, and four color image isfused onto the recording medium 32 at fuser 40.

The FIG. 3 single pass color printer provides color output at higherspeeds than the multipass design illustrated in FIG. 1. As a practicalmatter, however, it is difficult to achieve the necessary registrationof images for the FIG. 3 apparatus because of the exacting alignmentaccuracy required to obtain registration of the images created atsuccessive ones of the printing stations. To obtain acceptable levels ofregistration for the different image components of the FIG. 3 design, itis necessary to arrange the four different laser beam scanning units58-64 so that the laser beams trace lines on the surface of the drumthat are parallel to the cylindrical axis of the drum to a very highdegree. Because this alignment requires precise positioning of fiveobjects in relation to each other in three dimensional space, it is verydifficult to achieve an acceptable level of alignment, so that the FIG.3 apparatus is not amenable to high volume manufacturing techniques. Thedifficulty of aligning the four laser beam paths is heightened becausethe lasers are typically scanned using high speed rotating polygonmirrors, with cach mirror rotating independently of all others. Smallvariations in mirror position will thus be magnified by the long pathtraced by the laser beam reflected from the mirror to the surface of thedrum.

A further difficulty with the apparatus of FIG. 3 is that it tends to belarge. The drum 10 must be made sufficiently large so that the fourdifferent print stations and the transfer charger 30 can be arrangedabout its periphery. Thus, it is difficult to make the apparatus of FIG.3 in a small enough form factor to comfortably fit into the home andsmall office operating environments. One attempt to address this issueis illustrated in U.S. Pat. Nos. 5,541,722 and 5,557,394, which modifythe design of the FIG. 3 apparatus by providing light emitting diodearrays as optical exposure units within a transparent drum that has aphotoconductor formed on its surface. While this modification achievesreduced size, it requires the use of a transparent drum, which isgenerally undesirable for reasons of both cost and performance.Additionally, the design strategy of these two patents is limited in itspotential for success because the modifications do nothing to reduce thespace required by the developing units, which typically occupy a farlarger volume than the optical exposure units.

It is thus desirable to provide an improved configuration for anelectrophotographic apparatus. An associated, but distinct considerationin the design of high performance, low cost electrophotographic systemsis the desirability of providing a more compact and higher performancedeveloper. To appreciate this design consideration, it is useful toconsider the design of a conventional developing unit used in someelectrophotographic processes, illustrated in FIG. 4. The developingunit of FIG. 4 generally comprises a conductive roller that transports adeveloper consisting of a mixture of magnetic carrier particles andplastic or other toner ink particles to the surface of a photoconductivedrum which carries a latent image. The developing unit includes roller,stirrers or other mechanisms to agitate the developer. The agitateddeveloper adheres to the surface of the roller and a predeterminedthickness of developer is maintained on the roller by use of a doctoringblade. The toner is triboelectrically charged and the toner and carrierare carried from a reservoir within the developing unit to a positionadjacent the photoconductive drum by the roller as the roller rotates inan opposite sense to the photoconductive drum. A developing bias from ad.c. power supply and/or an a.c. power supply is applied between thephotoconductive drum and the developing unit to form an electrical fieldthat transports the developer from the roller to the photoconductivedrum.

FIG. 4 shows an exemplary structure of a conventional developing unit inwhich a tribocharging blade 80 and a feed roller 82 are providedadjacent to a developing roller 84 within a housing 86. Thetribocharging blade 80 is held against a surface of the developingroller 84, for example, by a spring mechanism. A stirrer 88 is providedwithin the reservoir of the developer mixture of carrier and toner 90.During developing, the toner 90 is agitated by the stirrer 88 and thendistributed on a surface of the feed roller 82. Next, the toner 90 onthe surface of the feed roller 82 is conveyed to the surface of thedeveloping roller 84 by the feed roller. At the same time, a strongshearing force between the tribocharging blade 80 and the developingroller 84 causes the toner 90 to be tribocharged. Subsequently, thetoner 40 is selectively transferred to a surface of a photoconductor 10by the electrostatic mechanism discussed above, thereby achieving thedeveloping operation.

In the conventional developing unit discussed above, there are a rangeof problems observed:

(1) The load caused by the frictional force between the tribochargingblade 80 and the developing roller 84 is large and the fluctuation inthat load can be dramatic, resulting in variations in the charging ofthe toner 90.

(2) A nip between the tribocharging blade 80 and the developing roller84 is small, introducing further instability to the tribocharging of thetoner.

(3) Toner particles arc crushed by the strong shearing force between thetribocharging blade 80 and the developing roller 84, resulting in afogging effect and reducing the image quality.

(4) The reservoir for storing the toner is separated from the otherdeveloping mechanisms. The configuration of the developing unit tends tobe complicated, resulting in wasted space and in residual toner beingleft in the developing unit that cannot be accessed in developingoperations.

It is therefore desirable for a developing unit to have a smallervolume, improved integration and improved performance as a part of anoverall strategy to facilitate the production of a high performance,readily manufactured color electrophotographic printing apparatus.

SUMMARY OF THE PREFERRED EMBODIMENTS

It is an object of the present invention to provide a simple and compactdeveloping unit which has improved performance.

It is another object of the present invention to provide a developingunit which has comparatively low internal loading and which can smoothlyand stably charge the contents of the developing unit.

It is another object of the invention to provide a developing methodwhich does not apply an unacceptable level of shearing force to tonerand which provides sufficiently undamaged toner as to produce highquality images.

Particularly preferred embodiments of the present invention provide atransportation device within the developing unit, such as a mesh belt,for moving toner within the developing unit. For such embodiments, it ispossible to achieve charging of the toner by causing the belt or othertransportation belt to carry the toner between opposed friction plates.

One aspect of the present invention provides a developing unit of thetype that might be used for electrophotography having a housing forenclosing components of the developing unit, the housing having anopening through which toner can pass. First and second rollers areprovided in the housing with a belt extending over the first and secondrollers so that the belt is translated around the first and secondrollers in response to rotation of the first and second rollers. Thebelt is capable of transporting toner on a surface of the belt. Acharging device is provided adjacent at least a portion of the belt, thecharging device imparting a charge to toner on the belt passing adjacentthe charging device. An output device transports toner from the surfaceof the belt to the opening in the housing.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will become apparent to those skilled in the art uponconsideration of the following description of the preferred embodimentsof the present invention taken in conjunction with the accompanyingdrawings.

FIG. 1 is a schematic view showing a conventional multipass colorelectrophotographic apparatus.

FIG. 2 is a schematic view showing a conventional tandem colorelectrophotographic apparatus.

FIG. 3 is a schematic view showing a conventional single pass colorelectrophotographic apparatus.

FIG. 4 illustrates a conventional developing unit that might be used inconjunction with one or more of the electrophotographic systems shown inFIGS. 1-3.

FIG. 5 shows a color electrophotographic apparatus according to thepresent invention.

FIG. 6 schematically shows one of the line memories used in storing datato be used in modulating the optical exposure mechanism.

FIG. 7 shows another color electrophotographic apparatus according tothe present invention.

FIGS. 8-10 show different embodiments of a developing unit in accordancewith the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides an electrophotographic apparatus thatproduces color hard copy output, preferable in a single pass process andmost preferably using a highly manufacturable and compact configuration.In one aspect, the present invention provides an electrophotographicsystem having a photoconductor with a planar surface along at least partof its path. The associated laser beam scanning units or other opticalexposure units for the electrophotographic system are positioned so thatthe exposure light from each of the optical exposure units extendsperpendicularly to the planar surface of the photoconductor. Bypositioning all of the optical exposure units so that their lightoutputs are incident on the planar portion of the photoconductor,alignment of the optical exposure units requires only that the units beadjusted so that their output beams are parallel to one another. This isa much simpler alignment problem than is presented by the single passelectrophotography system illustrated in FIG. 3. In aligning the systemof FIG. 3, it is necessary to precisely select the four distinct planestraced by the four beam scanning systems in a three dimensional spaceand to position those four planes with respect to a cylindrical surface.Aligning electrophotographic systems formed in accordance with thepresent invention requires only that four optical exposure mechanisms bepositioned so that their optical output beams trace parallel lines onthe surface of the planar portion of the photoconductor. From one pointof view, alignment of systems in accordance with this aspect of theinvention is like solving a two-dimensional problem, while the alignmentof the FIG. 3 apparatus requires solution of a harder three-dimensionalproblem. Those skilled in the art of optics and alignment of opticalsystems will recognize that making the four or more exposure beams ofpreferred embodiments of this invention trace parallel optical paths isa much simpler task than is involved in aligning the FIG. 3 system. Inaddition, such preferred embodiments will have far greater stabilitythan conventional systems like the FIG. 3 apparatus and so will tend toretain their alignment better. Thus, preferred embodiments of thepresent invention are expected to not only be less expensive tomanufacture but also to have better durability.

Several other aspects of the present invention facilitate theimplementation of a photoconductor having a planar surface inconjunction with four exposure units disposed to output parallel beams.One of these aspects is the use of a smaller developing unit. In thesystem illustrated in FIG. 3, the developing units occupy large volumesas a natural consequence of the design of these developing units. Thus,it is likely difficult to modify the drum of the FIG. 3 apparatus toprovide a planar surface on which images can be formed without makingthe apparatus undesirably large. By using a more efficient and compactdesign for the developing unit, preferred embodiments of the presentinvention accommodate the use of a planar configuration for thephotoconductor and other parts of a print engine. Briefly, particularlypreferred embodiments of certain aspects of the present inventionprovide a developing unit having a mesh or web belt for the internaltransport of toner within the developing unit, allowing for differentconfigurations of developing units to be used which more efficiently usespace. The use of a more active toner transport mechanism also allowsthe shape of the developing unit to be selected to optimize theintegration of the overall system. The developing unit can be made stillmore compact in some embodiments by using a single component developer,in contrast to the more conventional multicomponent developer. If thedeveloper has only toner particles and no carrier particles, thereservoir for developer will be used more efficiently, allowing thereservoir to me made smaller and thus allowing the developing unit to bemade smaller.

Another aspect of the present invention which facilitates the use of aplanar printing surface for the photoconductor is the use of highlyintegrated optical exposure units. In one configuration, the opticalexposure units might include four polygon mirrors that, along with theshaft on which the mirrors are mounted and which is used to rotate themirrors, are formed from a single piece of plastic in a process thatreadily produces mirror faces that are aligned and parallel to a highdegree. Thus, when four laser beams are scanned by this configuration ofoptical exposure unit, the four scanned beams trace through fourparallel planes with comparatively little alignment required. In anotherconfiguration, the optical exposure unit might include a rigid frame onwhich four LED arrays ("bars") are mounted. It is well within thecharacteristics of the LED manufacturing process and within typicalmechanical tolerances to align the LED arrays so that the LED arraysoptical output extends along parallel planes. These and otherconfigurations of optical exposure units for the present inventionfacilitate the planar configuration of the print engine of somepreferred embodiments of the present invention. In addition, theseoptical exposure units are readily manufactured and integrated with thesystem to make the manufacture of the entire electrophotography systemless expensive.

These and other aspects of the present invention are now discussed withparticular reference to certain preferred embodiments and with referenceto the figures. Implementations of the present invention use manycomponents which are commercially available or are otherwise well known.As such, detailed descriptions of these components are not provided hereso that the description is more concise and better emphasizes thedistinctive aspects of the invention's electrophotographic apparatus. Itshould be appreciated that the following description emphasizes a fourcolor laser printer, but other electrophotographic systems might alsobenefit from the principles of the present invention. For example, acolor copier based on scanned images which are separated into differentcolor components stored in different memories would readily implementaspects of the present invention. In addition, the present invention isnot limited to four colors. Embodiments of the present invention readilyincorporate additional (or fewer) printing stations with only simplemodifications.

FIG. 5 shows a color electrophotographic apparatus according to thepresent invention. As shown, the color electrophotographic apparatuscomprises flexible photoconductor 100 extending as an endless beltaround a roller set including a first roller 102 and a second roller104. The rollers 102, 104 are spaced apart by a sufficient distance thatthe planar surface defined on one side of the photoconductor 100 islarge enough to allow the various print stations to be positioned alongthat planar printing surface. Stability of the planar printing surfacecan be maintained by positioning a tensioning plate on the oppositesurface of the photoconductor between the rollers. Further improvementin the stability of the planar printing surface can be provided bypositioning a plate adjacent to and behind the planar printing surface.The lateral position of the photoconductor 100 is maintained by one ormore limiting pins placed on one or more of the rollers 102, 104 toprevent lateral movement of the photoconductor 100 during rotation. Inthe illustrated embodiment, the rollers are driven, or one of therollers is driven, to translate the photoconductor through the printingregion continuously in a downward direction. The translation directiondefines an ordering for the elements within the four illustrated printstations. The print stations include chargers 106-112 positioned as thefirst element of each of the print stations. Four developing units114-120 are interposed between the chargers 106-112. Four laser beamscanners are provided for the four print stations which are formed ofsemiconductor lasers 122-128 for emitting laser beams 130-136 that passthrough gaps between the chargers 106-112 and the correspondingdeveloping units 114-120 and reach the planar printing surface of theflexible photoconductor 100. The illustrated electrophotographic systemalso includes a conventional transfer charger 138, and, for example, aconventional fuser 140 including a heating roller 142 and a pressingroller 144. The chargers, laser beam scanners, and developing unitstogether make up four distinct print stations for providing (in orderalong the direction of rotation) yellow (Y), magenta (M), cyan (C) andblack (K) toner image components to the photoconductor.

According to a particularly preferred embodiment of the presentinvention, the flexible photoconductor 100 comprises a belt-shapedflexible substrate coated with an organic photoconductor. The substrateof the belt is typically chosen to be highly conductive and mightconsist of a metal mesh or web or a plastic coated with a film of metalso that the substrate can act as a ground plane for the photoconductor.When the substrate of the belt is formed from a plastic, it may also bedesirable to choose a conductive plastic, so long as the plasticprovides the desired flexibility characteristics. The conductivesubstrate of the belt is coated with a photoconductive material such asan amorphous semiconductor or an organic photoconductor. Often, organicphotoconductors are preferred because they tend to be more flexible andhave a longer history of use. The photoconductor preferably issubstantially insulative so that charge provided onto the surface of thephotoconductor remains on the surface of the photoconductor forsufficient time to pass through a print station, so long as thephotoconductor in maintained in the dark. It is also desirable for thephotoconductor to be rapidly discharged upon application of light of thewavelength and intensity of the laser beams 122-128. Organicphotoconductors are commercially available from, for example, EastmanKodak Company and Mitsubishi Chemical Corporation. Of course, otherphotoconductors having similar flexibility and photoconductiveproperties can be used.

Each of the laser beam scanners is associated with one of the polygonmirrors and with the well known focusing and collimating optics usedwith semiconductor lasers within laser printers. A shaft 158 passesthrough the aligned central axes of the polygon mirrors 150-156 and isdriven by a motor 160 to rotate the polygon mirrors. Alternately, ifhigher rotational speeds are desired, the shaft may be rotated on an airbearing as is known in the electrophotographic arts. The illustratedmirrors 150-156 might each have a square cross section and the entireassembly might be formed from a single piece of plastic. In onesimplistic approach, the mirror assembly might be formed from a startingrectangular prism block of plastic having a uniform, square crosssection by turning the block using a lathe to define the shaft portionsbetween the mirrors. The mirror faces would then be rendered reflective,for example, by sputter coating the faces with aluminum and then themirror assembly is ready for use. The resulting mirror assembly willhave four polygon mirrors held in fixed alignment with parallel andaligned faces. More practically, the mirror assembly can be formed byinjection molding of plastic followed by coating the mirror surfaceswith a metal film such as aluminum. Such a molding process can beaccomplished with sufficient accuracy to ensure that the mirror faceswill be aligned and parallel within the required tolerances.

The use of a fixed mirror assembly or optical exposure unit facilitatesthe alignment of the FIG. 5 electrophotographic system, since alignmentcan be accomplished by properly positioning the mirror assembly withrespect to the planar printing surface of the photoconductor and thenperforming relatively simple focusing and alignment operations for eachof the semiconductor lasers 122-128. Therefore, the array of fourparallel laser beams 130-136 emitted from the laser beam scanners canscan the surface of the photoconductor 100 in a direction perpendicularto the moving direction of the photoconductor 100 as they are reflectedby the rotating polygon mirrors 150-156 with a high degree of accuracy.

As is known in the art, including the art discussed in the backgroundabove, each of the lasers 122-128 is connected to a modulating circuitthat modulates the lasers with the information necessary to createyellow, magenta, cyan and black image components, respectively. Thecommercially implemented modulation schemes of available color laserprinters are in most regards adequate for use with the present systemand so will not be described further herein. On the other hand, certainmodifications to the known modulation schemes facilitate the assemblyand alignment of the FIG. 5 system. Vertical registration between thesuccessive yellow, magenta, cyan and black image components is obtainedby varying the timing between the respective modulation patterns on thebasis of the transport speed of the photoconductor 100. In particularlypreferred embodiments of the present invention, the delays between thesuccessive color image components are programmed into nonvolatilememories (EPROM, EEPROM or flash) at the time of manufacture so that theregistration can be empirically determined for each printer. Thus, aninitial alignment of the system is accomplished and then testing isperformed to determine alignment between successive color imagecomponents. This testing might be accomplished by printing testpatterns, as is known in the art. Adjustments in the relative delaysoptimize the resignation between successive color image components andthen these optimized delays are stored in the nonvolatile memories. Bythis strategy, registration with one half pixel or dot accuracy isaccomplished between all four color image components during the initialset up of the FIG. 5 system.

Horizontal registration can be optimized in a similar manner. The dataused to generate the modulation signals used to drive cach of the lasers122-128 are stored in line memories corresponding to each of the lasers.Four different line memories are provided for the respective lasers andcolor image components to be generated and each of the line memories isconfigured as illustrated in FIG. 6 with a preshift register 170, a linememory register 172 and a postshift register 174. The generalrepresentation of the line memory shown in FIG. 6 is appropriate whetherthe modulation data are single bit data stored in a shift register orare data words stored in an array of shift registers. By use of theillustrated register array, the desired modulation data an be shifted tothe right or to the left in the memory to advance or delay themodulation signal with respect to the other line memories. The advanceor delay of each horizontal line of each color image component can thusbe adjusted to obtain better horizontal registration between all of thecolor image components. In particularly preferred embodiments of thepresent invention, the relative advance and delay of each of the linememories is adjusted during manufacture and stored, for example in anonvolatile memory, to obtain horizontal registration between thedifferent color image components with one half pixel or dot accuracy.

The image reproduction operation of the FIG. 5 color electrophotographicapparatus according to the present invention proceeds as follows. Atfirst, the rollers 102 and 104 are rotated to move the flexiblephotoconductor 100 and then the first charger 106 applies a voltage of-600˜-800 V (in this embodiment, for example, -700 V) to a leadingportion of the planar printing surface of the photoconductor 100. Whenthe uniformly charged surface of the photoconductor 100 passes through agap between the first charger 106 and the first developing unit 114, thefirst laser beam 130 modulated with information for the first colorimage component impinges upon the charged area of the photoconductor 100and discharges the photoconductor) in accordance with the informationcorresponding to the image to be printed. At this time, the voltage onthe photoconductor corresponding to a blank (toner will not adhere)portion of the latent image is about -700 V while the voltage on thephotoconductor corresponding to a non-blank (toner will adhere) portionis between ±150 and 0 V. Subsequently, when the charged area carryingthe first latent image passes by the first developing unit 114,developer containing yellow toner is conveyed by the first developingunit 114 to the charged surface of the photoconductor 100 carrying thelatent image, thereby developing the yellow image component. Next,magenta developer, cyan developer and black developer images are appliedto the surface of the photoconductor, each color in sequence using aseries of charging, discharging and developing operations like thatdescribed for yellow, so that a four color toner image is formed on theplanar printing surface of the photoconductor 100. A recording medium162, which is generally a sheet of pater, is transported to a spacebetween the roller 104 and the transfer charger 138. The Y, M, C and Ktoner images on the surface of the photoconductor 100 are transferred tothe recording medium 162 by means of an electric field produced betweenthe photoconductor and the recording medium by the transfer charger 138.Then, the toner images are fused by the high temperature and highpressure provided by the fuser 140, fixing the four color toner image onthe recording medium 162. Typically, the photoconductor belt is blanketdischarged and cleaned of remnant toner or other debris at a cleaningstation 164 positioned on a portion of the photoconductor 100 away fromthe planar printing surface. This cleaning station might also include atensioning unit for maintaining an appropriate tension on thephotoconductor during its transport.

In the embodiments of the present invention illustrated by FIG. 5, laserbeam scanners having semiconductor lasers scanned by polygon mirrors areused. However, other light sources such as light emitting diode LEDs) ora light source modulated by liquid crystal devices (LCDs) can beutilized instead. FIG. 7 shows a color electrophotographic apparatusaccording to a second embodiment of the present invention wherein thelaser beam scanners in the first embodiment are replaced withindependently controllable arrays of LEDs or light sources modulated byLCDs 180-186. In this case, the semiconductor lasers 122-128, thepolygon mirrors 150-156 and the associated optics in the FIG. 5embodiments can be omitted. Therefore, in comparison with theelectrophotographic apparatus according to the first embodiment, theapparatus of the second embodiments is advantageous in that the spatialvolume thereof is smaller and the manufacturing cost is lower.Alternately, the developing units might be made larger to provideadditional toner capacity. The LED arrays LCD modulated light sources180-186 are mounted to a single rigid frame so that the optical exposureunits 180-186 are held in fixed relationship to one another. Mostpreferably, the LED arrays are imaged onto the planar printing surfaceof the photoconductor 100 using cylindrical optics (such as fiberlenses) which extend across the entire LED array. Other optics might benecessary when using LCD modulation, depending on the nature of thelight source. The optical exposure units 180-186 can be mounted to therigid frame so that the optical output of the array on averagepropagates along parallel planes in a direction perpendicular to theplanar printing surface of the photoconductor. As such, the opticalexposure units 180-186 present similar levels of alignmentsimplification over the conventional system of FIG. 3 as are provided byuse of the mirror assembly used in the FIG. 5 system.

As discussed above, the configuration of the electrophotographic systemsof FIG. 5 and FIG. 7 is facilitated by use of a reduced volumedeveloping unit. Such a reduced volume developing unit is provided inaccordance with a different aspect of the present invention. Whilepreferred embodiments of the present invention's developing unit findparticular application in the systems illustrated in FIGS. 5 and 7, itshould be appreciated that embodiments of the developing unit of thepresent invention can be used in other configurations of these and otherelectrophotographic systems. FIG. 8 shows a developing unit according toan embodiment of the present invention including a first roller 200, asecond roller 202, a transportation device 204 which is typically a meshor similar material, a friction device 206 positioned around a frictionregion 208 of the transportation device, toner 210 within a reservoir, adeveloping roller 212, and a thickness controlling blade 214. The FIG. 8developing unit is provided within a housing 216. The illustratedtransportation device 204 is a mesh or web belt stretched between thefirst roller 200 and the second roller 202, where the opening in themesh or web of the belt are large enough that toner can readily passthough the openings on the surface of the belt. The first roller 200 isconnected to a motor during operation to serve as a driving roller forthe transportation device 204 and as a feed roller for conveying chargedtoner to a surface of the developing roller 212. The second roller 202is used to restore the transportation device 204 in the illustratedembodiment.

The toner 210 carried by the transportation device 204 enters and passesthrough the friction device 206 before the toner reaches the developingroller. The friction device 206 includes two plates that sandwich thetransportation device 204, with the plates being formed from a materialselected so that the toner 210 is charged as it passes between theplates of the friction device 206. In some embodiments, the plates aremetal and are connected to a bias V_(B) which typically has a value of,for example, between about -300 to -700 V. Alternately, the chargingplates might be formed from a dielectric (nonconductive) material suchas synthetic fiber, paper, acrylic resin and the like. In this case, nobias is applied to the friction device 206 and charge is supplied to thetoner solely by tribocharging.

An important aspect of the FIG. 8 developing unit design is that thecharging plates of the friction device 206 face on the transportationdevice 204 over a large width and over a considerable length. Thisallows for high levels of tribocharging while imposing a relativelylight and constant load on the transportation device. The charged toner210 is conveyed to the first roller 200 and then from the transportationdevice 204 to a surface of the developing roller 212. The thicknesscontrolling blade 214 exerts pressure on the developing roller 212 tocontrol the thickness of the toner layer carried on the developingroller 212. The thickness of the toner layer controlled by the thicknesscontrolling blade 213 is between 10 microns to 100 microns on thesurface of developing roller 212. The developing roller 212 faces on thephotoconductor 220 though a window 222 and the window may be controlledto selectively eject toner 210 onto photoconductor 220.

Provision of the transportation device 204 for moving toner through thedeveloping unit allows the developing unit of FIG. 8 to be made smallerand/or shaped differently than the conventional developing unit. Thetransportation device 204 also allows the FIG. 8 developing unit to bemade without a stirring mechanism. In addition, the transportationdevice allows a single component developer, that is, a toner ink withoutcarrier, to be used in the developing unit, allowing the developing unitto be made smaller. In presently preferred embodiments of the invention,the transportation device is a mesh belt formed from a conductivematerial. Each of the toner particles has a diameter ranging fromseveral microns to tens of microns. The mesh of the transportationdevices 204 allows the toner particles to pass though, and the openingsin the mesh preferably range from tens of microns to hundreds ofmicrons.

FIG. 8 shows a developing unit according to another embodiment of thepresent invention generally similar to the developing unit shown in FIG.8 except that a different configuration of a friction device 230 is usedinstead of the friction device 206 and the second roller 202 of the FIG.8 embodiment. In the FIG. 9 embodiment, a transportation device 204 isdriven by a first roller 200, and the friction device includes acylinder 230 and a hemispherical shell 232, with the transportationdevice 204 running between the cylinder 230 and the hemispherical shell232. The friction device stretches the transportation device 204 tomaintain an appropriate tension on the transportation device and alsotribocharges the toner. The cylinder 230 rotates as the transportationdevice 204 is move so that the toner carried by the transportationdevice 204 is tribocharged by the hemispherical shell 232. Other aspectsof the developing operation using the FIG. 9 apparatus are the same aswere describes with respect to the FIG. 8 first embodiment, and so thedescription of those aspects is not repeated.

FIG. 10 shows another variation on a developing unit according to thepresent invention and having a structure substantially the same as thatof the FIG. 8 embodiment except that a "fur" or soft roller 230 is addedbetween the first roller 200 and the developing roller 212. The surfaceof the fur or soft roller 240 might, for example, consist of a polyesteror other fiber having a nap on the order of about 0.5-2.0 millimeters.Toner is stored temporarily within the nap on the soft roller as anintermediate reservoir which ensures that there will be a substantiallyconstant supply of toner available to the developing roller 212. Toner210 is carried by the transportation device 204 and tribocharged by afriction device 206. Next, the charged toner 210 is conveyed to a firstroller 200 and then to the soft roller 240. The charged toner is fedfrom the soft roller 240 to a surface of a developing roller 212, andthen provided to the photoconductor 220 via a window 222.

In each of the above embodiments of developing units illustrated inFIGS. 8-10, the contact area of the friction device is large and anendless mesh belt is used to carry toner as it is charged. Consequently,the toner carrying operation is smooth, the load of the developing unitis stable and the power consumption of the developing unit is reduced.Further, reduction in the driving load of the developing unit decreasesthe size and weight of the developing unit, and the manufacturing costis also decreased. In addition, since the forces applied to tonerparticles are reduced, less damage to the toner particles occurs so thatthe image quality is improved.

The various aspects of the present invention have bee described in termsof certain presently preferred embodiments. Those of ordinary skill willappreciate that modifications to and variations from the preferredembodiments might be made while remaining consistent with the basicteachings of the present invention. As such, the scope of the presentinvention is not to be limited to the particular described embodiments.Rather, the scope of the present invention is to be determined from theclaims, which follow.

What is claimed is:
 1. A developing unit, comprising:a housing forenclosing components of the developing unit, the housing having anopening through which toner can pass; first and second rollers in thehousing; a belt extending over the first and second rollers, the beltbeing translated around the first and second rollers in response torotation of the first and second rollers, the belt capable oftransporting toner on a surface of the belt; a charging device adjacentat least a portion of the belt, the charging device imparting a chargeto toner on the belt passing adjacent the charging device; and an outputdevice for transporting toner from the surface of the belt to theopening in the housing.
 2. The developing unit of claim 1, wherein theoutput device comprises a developing roller for receiving toner conveyedby the belt and for providing toner to the opening in the housing. 3.The developing unit of claim 2, wherein the belt comprises openingsformed on the surface of the belt.
 4. The developing unit of claim 3,further comprising toner particles having a first average diameter,wherein the openings have an average diameter larger than the firstaverage diameter.
 5. The developing unit of claim 1, wherein thecharging device charges toner by triboelectric charging.
 6. Thedeveloping unit of claim 3, wherein the charging device comprises a pairof friction plates.
 7. The developing unit of claim 6, wherein thefriction plates are metal.
 8. The developing unit of claim 6, whereinthe friction plates are connected to a bias potential.
 9. The developingunit of claim 6, wherein the friction plates are nonconductive.
 10. Thedeveloping unit of claim 2, further comprising a soft roller disposedbetween the belt and the developing roller, the soft roller having a napon its surface capable of storing toner.
 11. The developing unit ofclaim 1, wherein the charging device includes a curved plate separatedfrom the second roller by the belt, the curved plate acting incooperation with the second roller to charge toner passing on the beltbetween the second roller and the curved plate.
 12. The developing unitof claim 11, wherein the curved plate is a hemispherical shell.
 13. Thedeveloping unit of claim 11, wherein the output device comprises adeveloping roller for receiving toner conveyed by the belt and forproviding toner to the opening in the housing.
 14. The developing unitof claim 11, wherein the belt comprises a mesh so that openings areformed on the surface of the belt.
 15. The developing unit of claim 14,further comprising toner particles having a first average diameter,wherein the openings have an average diameter larger than the firstaverage diameter.
 16. The developing unit of claim 11, wherein thecharging device charges toner by triboelectric charging.
 17. Thedeveloping unit of claim 11, wherein the second roller and the curvedplate are metal.
 18. The developing unit of claim 17, wherein the secondroller and the curved plate are connected to a bias potential.
 19. Thedeveloping unit of claim 16, wherein the curved plate is nonconductive.20. The developing unit of claim 13, further comprising a soft rollerdisposed between the belt and the developing roller, the soft rollerhaving a nap on its surface capable of storing toner.
 21. A method ofdeveloping used in an electrophotographic process, the methodcomprising:driving a transportation system including a belt having asurface for carrying toner particles; charging the toner particles bypassing the toner particles on the belt through opposing frictionplates; conveying the charged toner particles to a developing surface;and transferring the charged toner from the developing surface to asurface of a photoconductor.
 22. The method of claim 21, wherein thebelt is driven by a set of rollers over which the belt is stretched. 23.The method of claim 21, wherein the belt has a mesh surface havingopenings larger than the average size of the toner particles.